Sputtering technique

ABSTRACT

THE ELECTWRICAL AND PHYSICAL PORPERTIES OF CATHODICALLY SPUTTERED FILMS MAY BE CONTROLLED BY UTILIZING ALTERNATIVELY, ELECTRON NEGATIVE ION, AND POSITIVE ION BOMBARDMENT OF SUBSTRATE SURFACES DURING THE COURSE OF THE SPUTTERING PROCESS.

May'29, SCHWARTZ ET AL 3,736,242

SPUTTERING TECHNIQUE Original Filed Jan. 31, 1968 2 Sheets-Sheet 1 FIG.

FIG. 2 b.c.c.TANTALUM B TANTALUM Eu 250 -|600 g E -|400 E E 2 200- -1200 a g? I50 TCR l0OO 8 3 Lu 2 800 u U I 600 55; E i 00 400 E5 5 e 200 3 E 50 0 m 00 E SUBSTRATE POTENTIALVOLTS 3 TANTALUM I50 +-b.c.c.TANTALUl\/| 4 200 E 10005 2 6 g g 800 55 8 I 600 E 00 LL, o. Q. & IL 400 3 m 25 a /v SCHWARTZ 1 5 WVENTORSF. wmnvr CU RRENT-TH IRD ELECTRODE 0 .1 .2 .3 .4 5 .6 RATO OF CURRENT-SECOND ELECTRODE W f ATTORNFV United States Patent Int. Cl. C23c 15/00 U.S. Cl. 204-192 1 Claim ABSTRACT OF THE DISCLOSURE The electrical and physical properties of cathodically sputtered films may be controlled by utilizing alternatively, electron negative ion, and positive ion bombardment of substrate surfaces during the course of the sputtering process.

This application is a division of copending application Ser. No. 701,964, filed Jan. 31, 1968, now U.S. Pat. No. 3,589,994 of June 1971, which was a continuation-in-part of copending application Ser. No. 372,537, filed June 4, 1964, now abandoned.

The present invention relates to a technique for the deposition of thin films by cathodic sputtering techniques. More particularly, the present invention relates to a technique for the growth of thin films of controlled electrical and physical properties by a novel cathodic sputtering procedure.

In recent years interest has been expanded in thin film metal electrical resistors and the preparation of such films by cathodic sputtering techniques. Unfortunately, it has frequently been found that the specific resistivity and temperature coefiicient of resistance of certain of these films are prone to variability and non-uniformity. These difiiculties have been controlled to a limited extent by painstaking control of background pressures, leak rates and deposition parameters. However, variability is still found in the quality of the deposited film.

In accordance with the present invention, a technique for the deposition of thin films by cathodic sputtering is described wherein the electrical and physical properties of deposited films are controlled by utilizing alternatively, electron or negative ion, and positive ion bombardment of substrate surfaces during the course of the sputtering process. The inventive technique involves sputtering in a three electrode system including an anode member, a cathode member, and a ground or reference electrode, wherein the anode member is biased either with an alternating potential of at least 0.1 volt with respect to the reference electrode, the cathode being biased at conventional potentials with respect to the reference electrode. In the operation of the sputtering process, the substrate member is physically situated upon the anode member and assumes the potential thereof when a difference of potential is impressed between anode and reference electrode, thereby resulting in the desired bombardment of the substrate surfaces.

The described technique has been found to result not only in control of film parameters such as resistivity and temperature coefiicient of resistance but also in the con trol of physical properties, namely, crystallographic phase and chemical composition. Thus, for example, sputtering of tantalum in accordance with the inventive procedure may be effected to yield either beta tantalum or body centered cubic tantalum of predetermined resistivity and temperature coeflicient of resistance, a signi- :ficant advantage from a device standpoint. Furthermore,

the degree of film perfection is susceptible to control by the described technique. g

In an alternative embodiment, it has been determined that the technique is equally efficacious inQa reactive ambient. Therefore, cathodic sputtering of thin films in the presence of nitrogen, oxygen, and so forth, may also be effected by operation in the described manner.

The described technique departs from conventional diode sputtering by the use of a reference electrode maintained at ground or a fixed potential and with respect to which both the anode and cathode members are biased.

For convenience, the electrode system described herein will hereinafter be designated as follows:

First electrode=reference electrode Second electrode=anode Third electrode-:cathode It will also be understood by those skilled in the art that in the operation of the process, the substrate member upon which the thin film of interest is to be deposited is physically situated upon the second electrode and assumes the potential thereof.

The invention will be more readily understood by reference to the following detailed description, taken in conjunction with the accompanying drawing wherein:

FIG. 1 is a front elevational 'view, partly in section, of an exemplary apparatus suitable for the practice of the present invention;

'FIG. 2 is a graphical representation on semi-log coordinates of specific resistivity in microhm-centimeters and temperature coefiicient of resistance in p.p.m./ C. against applied substrate potential in volts showing variations in the noted parameters and structure as a function of anode bias potential for sputtered tantalum films prepared in accordance with the present invention;

FIG. 3 is a graphical representation on linear coordinates of specific resistivity in microhm-centimeters and temperature coefficient of resistance in p.p.m./ C. against the ratio of applied alternating current and cathode DC current showing variations in the noted parameters as a function of anode-cathode current ratio and structure for sputtered tantalum films prepared in accordance with the present invention;

FIG. 4 is a graphical representation on semi-log eoordinates of percent relative imperfection against applied substrate potential in volts showing variations in film perfection as a function of substrate bias for sputtered b.c.c.-tantalum films prepared in accordance with the present invention; and

FIG. 5 is a graphical representation on semi-log coordinates of specific resistivity against oxygen partial pressure showing variations in resistivity as a function of oxygen partial pressure for reactively sputtered tantalum films prepared in an oxygen ambient in accordance with the present invention, the substrate having applied positive potentials of 0.1, 5, and 50 volts D-C.

With reference now more particularly to FIG. 1, there is shown a vacuum chamber 11 provided with an outlet 12 for connection to a vacuum pump (not shown), an inlet 13 for the introduction of a suitable sputtering gas, and a base plate 14 which acts as the first electrode for sputtering. Shown disposed within chamber 11 is a substrate holder or second electrode 15 and a third electrode 16, the latter being comprised of the material which is required to be deposited upon substrate member 17. Third electrode 16 is connected to the negative pole 18 of a direct current high potential supply, the positive pole of which is connected to base plate 14 (as at 19), the latter being connected to ground bias or permitted to float. Second electrode 15 may be connected to a connected 6115555151614 t at 23),

The present invention may conveniently be described by. reference toanillustrative example wherein it is desired. to-camoaiesn sputter any of the well-known film-form ing metals, for example, tantalum, niobium titanium, zirconium, aluminum, etcetera, in an apparatus of the type sh wn min '1. 1 Subst'rate, 17 is first vigorously cleaned, Conventional cleaning agents are suitable for this purpose, the ghoice of a particular one being dependent upon the composi tion ofithe substrate itself. Substrate 17 is placed upon substrate omer 1 5 as shown in FIG, l, the latter being composed: of a suitable conductor, for 'example, tanta1um, stainless steel, et cetera. 1 The. vacuum. techniques utilized in this invention are known (see Vacuum Deposition of Thin Films, L. Holland, J. Wiley & Sons, Inc., New York, 1956). By this process the vacuum chamber is first evacuated, flushed with an inert gas, as, for example, any of the members of the rare gas family such as helium, argon or neon, and the chamber then re-evacuated. The extent of the vacuum required is dependent upon consideration of several factors which are well known to those skilled in the art. However, for the purposes of the present invention, a practical initial pressure range is l to 10- torr., while suitable inert gas pressures during sputtering range from 0.5 to 100x10 torr.

After the requisite pressure is attained, third electrode 16, which may be composed of any of the above-noted film-forming metals or, alternatively, may be covered with any of the film-forming metals, for example, in the form of a foil, is made electrically negative with respect to base plate 14.

The minimum voltage necessary to produce sputtering is dependent upon the particular film-forming metal employed. For example, a direct current potential of approximately 1000 volts may be employed to produce a sputtered layer of tantalum suitable for the purposes of this invention, minimum voltages for other film-forming metals being well known to those skilled in the art. However, in certain instances it may be desirable to sputter at voltages greater than or less than the noted voltage.

The next step in the inventive procedure involves ap plying a potential to substrate holder and substrate 17 whereby they are made electrically positive with respect to base plate 14. This end may be attained by applying an alternating current potential to holder 15 by conventional means.

It has been determined that an alternating current potential ranging from 0.1 to 5000 volts may be applied to the ungrounded'substrate holder with respect to the base plate.

The spacing between the substrate holder (anode) and cathode is not critical. However, the minimum separation is that required to produce a glow discharge. For the best efficiency during the sputtering process, the substrate should be positioned immediately without the well known Crookes Dark Space.

The balancing of the various factors of voltage, pressure and relative positions of the cathode and substrate holder to obtain a high quality deposit is well known in the sputtering art. However, it will be appreciated that the main impact of the present invention lies in the discovery that applying a specific bias to substrate holder 15 and substrate 17 during sputtering permits control of film' parameters.

With reference now more particularly to the example under discussion, by employing a proper voltage, pressure and spacing of the various elements within the vacuum chamber, alayer of a film-forming metal is deposited upon substrate 17. The sputtering is conducted for a pe riod of time calculated to produce the desired thickness.

With reference now more particularly to FIG. 2, there is shown a graphical representation on semi-lo coordinates ofresistivity in micro hm centimeters and temperature, co-v efiicie nt of resistance in p.p,m./ C. against applied po- It is noted that with the substrate holder essentially at zero potentialtindicated as +0.1 volt in the logarithmic voltage scaleof the figure) the deposited film evidenced a resistivityijof approximately. 200 microhm-centimeters and a temperature coefiicient of resistance of approximatevolts results in the formation of beta tantalum, whereas increasing the bias beyond 10 volts results in the formation of bo centered cubic tantalum.

With reference now to FIG. 3, there is shown a graphical representation on linear coordinates of resistivity in microhm-centimeters and temperature coefficient of resistance in p.p.m./ C. against anode cathode current ratio showing the properties obtained by sputtering tantalum in the apparatus shown in FIG. 1 employing a cathode potential of 5000 volts direct current, and argon pressure of 10 millitorr and various anode to cathode current ratios wherein an alternating bias is applied to the anode.

It is noted that the use of an anode to cathode current ratio of 0.2 results in the deposition of a film evidencing a resistivity of approximately 42'microhm-centimeters and a temperature coefiicient of +1000 p;p.m./ C. As the anode to cathode current ratio increases, the resistivity is increased while the temperature coefficient decreases rapidly whereas decreasing the anode to cathode current ratio results in a similar trend.

It will be appreciated, therefore, that it is possible to adjust the resistivity and temperature coefiicients as well as structure of the deposited films by appropriate substrate biases.

- With reference now to FIG. 4, there is shown a graphical representation of relative film imperfection (relative internal stress in the film) as a function of substrate potential for bee-tantalum films deposited in accordance with the invention. As noted, with the substrate essentially at zero potential (indicated as :01 volt in the logarithmic voltage scale of the figure) the degree-of perfection is approximately 25 percent, and with increasing positive bias the degree of perfection'decreases to tap e proximately 0 percent (perfection being determined by usuallycounting the number of holes and cracks in the film). However, negative biasing ultimately results in a dramatic increase in imperfection level to a high of 100 percent, at which point the films are of no interest from a device standpoint.

FIG. '5 is a graphical representation showing specific resistivity as a function of oxygen partial pressure in tort for trantalum films reactively sputtered in the presence of oxygen in accordance with the present invention, the substrate member being biased at substrate potentials of 0.1, 5, and 50 volts. It will be noted from the graph that over therange of oxygen partial. pressures of approximately 10* 10 torr the specific resistivity of the deposited films may be controlled over a range of from 30 to 10,000

- microh'm-centimeters at the various applied biases. Additionally, it has been found that the temperature coetficient of'resistance of certain ortions of tbe'described curves are within the range from 150 to 300 p.p.m./ C., a desirable property from a device standpoint.

Several examples of the present invention are described in detail below. These examples are included merely to aid in the understanding of the invention, and variations may be made by one skilled in the art without departing from the spirit and scope of the invention.

EXAMPLE I This example describes the preparation of a sputtered tantalum film.

A cathodic sputtering apparatus similar to that shown in FIG. 1 was used to produce tantalum films. In the apparatus employed, the base plate 14 was grounded, the cathode was biased at 5 kilovolts negative with respect to ground, and the substrate holder and substrate were biased volts positive with respect to ground.

A glass microscope plate was used as the substrate. The slide was washed in a nonionic detergent, boiled in hydrogen peroxide and dried. The tantalum was employed in the form of an arc melted ingot slab as the cathode of the apparatus.

The vacuum chamber was initially evacuated to a low pressure of the order of 10 torr, flushed with argon and re-evacuated to 10 millitorr.

The substrate holder and cathode were spaced approximately 3 inches apart, the substrate being placed on the former. A direct current voltage of 5000 volts was impressed between cathode and base plate, and a direct current voltage of +5 volts was impressed between substrate holder and substrate 17, and base plate '14.

Sputtering was conducted for 60 minutes, so resulting in a beta-tantalum film 5000 A thick and evidencing a resistivity of approximately 200 microhm-centimeters and a temperature coeflicient of resistance of approximately 50 p.p.m./ C.

EXAMPLE II The procedure of Example I was repeated with the exception that an alternating bias of 250 volts was im pressed between the ground and the substrate holder and substrate, so resulting in an anode-cathode current ratio of 0.2 and so resulting in a tantalum film evidencing a resistivity of approximately 42 microhm-centimeters and a temperature coei'ficient of resistance of 1000 p.p.m./ C.

What is claimed is:

1. A method for the deposition of thin rfilms of controlled electrical and physical properties upon a substrate cathodic sputtering in a vacuum chamber in which there is a first electrode, a second electrode having a substrate positioned thereon, and a third electrode which comprises the steps of evacuating the said vacuum chamber, admitting a sputtering gas into said vacuum chamber, applying an alternating bias to said second electrode and substrate at a potential of about 0.1 to 5,000 volts with respect to said first electrode and biasing the said third electrode negative with respect to said first electrode and said second electrode at a potential distinct from that of the said second electrode, thereby effecting sputtering at said third electrode, deposition of a thin film of sputtered material upon said substrate and electron bombardment and alteration of the properties of said thin film.

References Cited UNITED STATES PATENTS 2,164,595 7/ 1939 Siebertz 204-192 3,021,271 2/1962 Wehner 204192 3,258,413 6/1966 Pendergast 204192 3,394,066 7/1968 Miles 204164 3,324,019 6/ 1967 Laegreid et a1 204192 3,361,659 1/1968 Bertelsen 204192 3,391,071 7/1968 Theuerer 204-192 JOHN H. MACK, Primary Examiner S. S. KANTER, Assistant Examiner 

